1. Technical Field
The invention relates to a method of controlling a pressurized water nuclear reactor in which control bars in the core are repositioned when a monitoring or operating parameter representative of the difference between the current power of the core and the power demanded (such as the mean temperature of the core) departs from a given range, called "dead band", the repositioned bars being selected so as to avoid increasing any difference between the axial power offset and a reference value; the terms "axial power offset" designate the ratio between the difference of the neutron fluxes in the upper and lower halves of the core and the sum of the fluxes, or designate another parameter representative of the imbalance of the fluxes along the path of travel of the cooling water through the core.
2. Prior Art
A control method of the above-defined type is already known (EP-A-0,051,542 or FR-A-2,493,582). The control bars contain a material which highly absorbs neutrons without giving rise to fissile isotopes (hafnium, for example), in an amount such that the bars have an anti-reactivity or bar worth sufficient for load follow (usually about 1000 pcm when the bars are totally inserted in the core). The bars are generally formed by clusters of rods each having a sheath containing neutron absorbing material pellets.
EP-A-0,051,542 proposes a control law as follows:
if the operating parameter is outside the dead band, computing the direction and displacement speed to be given to a group of bars selected responsive to the axial offset (to avoid increasing the differential between the latter and the reference value) as a function of the value and of the sign of the operating parameter;
if the operating parameter is within the dead band, repositioning a group of bars to reduce the axial offset, only if the difference between the current value of the latter and the reference value exceeds a predetermined threshold, then compensating for the repositioning by varying the boron concentration.
The prior art method requires the use, in addition to the control bars (and shut-down bars which, in normal operation, are always removed from the core and are inserted to shut down the reactor and keep it shut down) of boron in the form of a compound soluble in the cooling water; the boron content is modified to compensate for the reduction of reactivity, due particularly to the progressive depletion of the fuel, and to greatly increase anti-reactivity in case of a serious accident. The initial boron content must consequently be very high. Often the boron content of the water is also varied to regulate the power of the reactor and more particularly to compensate for variations of the xenon effect, the control clusters being repositioned only to vary reactivity during rapid operating transients.
The use of boron dissolved in the water forming the primary coolant has advantages. The anti-reactivity which it introduces is distributed evenly throughout the core. On the other hand, it also has serious drawbacks. The presence of boron results in an appreciable production of liquid effluents which must be processed with a complex installation. Boric acid corrodes some materials, particularly the zirconium-base alloy sheaths.
Another disadvantage is related to the fact that it would be dangerous to rely solely on injection of boron for shut-down of the reactor in case of a serious incident since, due to its construction, the boron injection system has an appreciable time constant. If soluble boron is used as essential control element, it is nevertheless necessary for the absorbent bars to have sufficient anti-reactivity for an emergency shut-down of the reactor. Load follow-up requires the possibility of rapidly reducing the boron content. That becomes impossible when the boron content for normal operation has been reduced to a low value because of the depletion of the core.
Methods have also been proposed for reducing the maximum boron content required in the water forming the primary coolant and/or the variations in the boron content during operation. According to FR-A-2,547,447, groups of control bars are displaced and, possibly, the boron concentration in the primary circuit is modified when it is necessary to bring the reactor condition from a state .PHI. (expressing the actual power and axial distribution of the reactor) to a reference state .PHI.c, taking into account the results of a calculation. This calculation consists in determining the variation to be given to external parameters (particularly the position of the control clusters and the boron concentration) by an iterative calculation which involves predicting a coupling relation between the external parameters and the state of the reactor, taking into account internal uncontrollable parameters, such as the moderation coefficient. Once the coupling relation has been determined, optimum variations of the external parameters are determined to approximate the reference state .PHI.c. But FR-A-2,547,447 does not teach how the coupling relation can be determined and seems to take into consideration only the axial power distribution, whereas it is important not to neglect the radial power distribution, and particularly the risk of appearance of power peaks.
Another approach for reducing the required boron concentration variations to compensate fuel depletion, hence the initial boron content, consists in varying the energy spectrum of the neutrons during an operating cycle of the core, starting from an epithermic spectrum. FR-A-2,496,319 relates to such a method, in which "grey" bars, i.e., bars having a moderate neutron absorption, are removed as the core is depleted. The bars are then replaced by water which increases the moderation ratio and shifts the energy spectrum of the neutrons closer to the thermal range. The power of the reactor is controlled by means of "black" bars. To that end, the local neutron flux is measured in several zones of the core with a large number of fixed detectors and a combination of displacements of the bars is chosen which gives the required reactivity variation while disturbing to a minimum the power distribution profile, as a function of the power demanded. This method involves an extremely complex calculation, using data from a very large number of neutron flux detectors placed in the core, and yet this method does not completely overcome the problem of using boron as a soluble compound for controlling the reactor.
Still other methods for controlling nuclear reactors plants are known which attempt to minimize or decrease the difference between the actual axial offset and a prescribed value. Such a method is disclosed in European patent application No. 0,097,488.